This invention relates generally to the analysis of gases and deals more particularly with a method and apparatus which makes use of ultrasound to monitor the ratio of gases in a binary mixture such as a therapeutic air/oxygen mixture.
In the analysis of gas mixtures containing several different gases, complicated and expensive techniques such as mass spectrometry are often employed. For example, air pollution studies typically require complex methods of analysis involving numerous procedures for identifying and quantifying the gases present in a sample. Even the analysis of simpler gas mixtures normally requires the use of measurement techniques which are specific for the gases involved and which require frequent calibration, replenishment of reagent chemicals, and other awkward, costly and time consuming procedures.
Frequently, the objective is to continuously monitor the ratio of only two gases of known identity. In such cases, the measurement system need not be specific for the gases involved since their identity is known, and it is normally important for the equipment and technique to be simple, reliable and inexpensive. By way of example, oxygen/nitrogen mixtures are used in the therapeutic administration of oxygen from oxygen concentrators in home healthcare. Additionally, respirators, ventilators and air/oxygen blenders are commonly used in hospitals. There are other medical applications aside from the obvious ones in anesthesia. All of these involve what can be treated as binary mixtures.
Other important applications for binary gas analysis exist outside the field of medicine. For example, high altitude aircraft produce oxygen by using oxygen concentrators. Also, large volume oxygen concentrators are being increasingly used for welding, particularly in third world countries where delivery of tanked oxygen is difficult. Applications also exist in underwater vehicles and in some mines.
Of the oxygen monitors that are commercially available, the only ones presently feasible for the purposes recited above utilizes either a fuel cell or a polarigraphic half-cell, which not only makes these devices unduly expensive but also requires frequent cell and electrolyte replacement because they are rapidly consumed in a high oxygen environment. Both types of units are overly sensitive to changes in temperature and in general are poorly temperature compensated. They are also sensitive to changes in barometric pressure. Moreover, these monitors respond sluggishly to changes in gas composition, requiring typically one minute to equilibrate to large changes. For these and other reasons, they are poorly suited for use by lay people and/or in unattended applications.
Although it has long been known that acoustical techniques can be used for gas analysis, severe technical problems have been encountered when attempts are made to analyze gases by using acoustic waves. These problems are acoustical, mechanical, electronic and thermal, and they all contribute to the main problems caused by standing waves and temperature changes. In the past, continuous wave systems have been considered appealing because they permit the use of resonant transmitter and receiver elements, affording a good signal-to-noise ratio, high sensitivity and simplicity of design. However, the continuous-wave approach invites new problems, particularly within a closed transducer chamber. In particular, the transmitter element must have a mechanical resonant frequency that matches the driving frequency while the receiver element must be mechanically tuned to the anti-resonant frequency in order to provide the necessary low driving impedance and high receiving impedance. In a continuous-wave system, the receiver accepts acoustic energy from the transmitter within a transducer chamber and generates a signal, with phase shift affected by mean molecular weight and temperature of the gas, to the receiver circuit. The acoustical waves also reflect from the surfaces within the transducer chamber, thus setting up the standing waves that frustrate repeatable measurements. In addition, upon excitation by the transmitted energy, the receiver retransmits a signal at its anti-resonant frequency in a complex fashion back toward the transmitter, and the result is a beat-frequency that has an unpredictable effect on response to temperature, in addition to the other problems normally associated with standing waves.
Ceramic transducers, though simple, sensitive and inexpensive, have widely varying temperature coefficients. In addition to the normal sound-in-gas temperature factor that is mathematically predictable, there are unpredictable temperature induced shifts in transducer capacitance and mechanical resonant frequency. There are also other more subtle variables that contribute to standing waves. For all of these reasons, even though they are useful for approximate triggering of low oxygen alarms in oxygen concentrators, gas analyzers based on the continuous-wave technique have inadequate accuracy and repeatability for requirements that are more sophisticated, such as these served by the present invention.
Most prior inventors who have adopted the continuous wave technique have failed to even recognize the standing wave problem, and those who do recognize the problem have not been able to successfully solve it. For example, U.S. Pat. No. 2,963,899 to Martin proposes filling the transducer chamber with granular material that is permeable to gas to prevent standing waves. However, filling the chamber in this manner entraps gas in small pockets and markedly slows down the response of the system to change in gas composition. U.S. Pat. No. 3,848,457 to Behymer and Japanese Patent No. 52-36089 to Koki show units that tune the driving frequency in order to deliberately establish a standing wave and then note deviations in loops and nodes with changes in gas composition. Both U.K. Patent No. 798,323 to Lawley and U.S. Pat. No. 4,220,040 to Noguchi recognize and directly address the standing wave problem. However, only Noguchi proposes a workable solution by damping the receiver and transmitter by way of "negative emittance amplifiers." These are essentially emitter-coupled flip flops which are said to present a short circuit across both transducer elements and thus damp them.
Another ultrasound technique used in the past is exemplified by U.S. Pat. No. 3,557,605 to Lanneau and U.S. Pat. No. 3,798,528 to Molyneuz. In this type of unit, a single electrical pulse is periodically applied to a nonresonant damped transmitter crystal, the acoustic impulse is delivered to a damped receiving crystal, and the transit time is measured. A variation of this is referred to as the "singaround" and is disclosed by U.S. Pat. Nos. 3,468,157 to Burke, 2,568,227 to Eligroth, 4,300,167 to Lorgini and 2,263,750 to Mikelson. The "sing-around" technique involves using the received impulse to retrigger the transmitter pulse, so that the frequency generated by this electro-acoustic loop varies with gas composition and temperature. Impulse methods avoid standing wave problems and thereby greatly improve the predictability of temperature behavior; however, they require the use of nonresonant transducer elements. They also lack the high sensitivity of the continuous-wave method, and they suffer a reduced signal-to-noise ratio due to the fact that transient noise spikes, such as those encountered in an environment with mechanically vibrating parts, can trigger false signals.
The effects of temperature on the speed of sound waves are well known. However, most of the prior attempts at monitoring gases have treated this phenomenon lightly and have failed to appreciate its considerable significance and complexity. U.S Pat. Nos. 3,557,605 to Lanneau and 4,425,804 to Mount control the temperature of the gas and/or its environment to a constant value, but in a compact and uncomplicated instrument this is difficult to do efficiently because of the inherently poor conductivity of the gas. Furthermore, the temperature of the sample gas that is introduced may differ considerably from that of the body of the transducer, as may occur within an oxygen concentrator. Another technique that has been proposed is to transmit ultrasound simultaneously through a standard gas as well as through the sample gas, as typified by U.S. Pat. Nos. 2,984,097 to Kniazuk, 3,353,400 to Schafft and German Patent No. 3,009,566. Comparing the transit times cancels the direct temperature influence but does not eliminate standing waves. This approach is accurate only when the temperatures of the two gases are equal and this is not always the case. For example, there can be a significant temperature differential encountered when ventilating an accident victim with a warmed air/oxygen mixture in a freezing environment.